add jdk8 for CI

.travis.yml

@@ -1,7 +1,9 @@
 #specific JDK or multiple JDK can be specified here. There is an issue with mvn surefire and openJDK8. 
 #It is better to switch to openJDK
 language: java
-
+jdk:
+  - oraclejdk8
+  - openjdk8
 
 #This helps to avoid downloading artefacts every time from MVN central. 
 #If you find any CI build failing due to unreflected dependency changes, kindly clear the CI cache